feat: album/playlist header and playlist ownership filtering

- Add TrackContextHeader widget: shows album art (fetched via NAM),
  title, subtitle (artist/description), and metadata (year · tracks · duration)
  above the track list when an album or playlist is opened
- Hide header for favorite tracks and search results
- Store user ID in AppSettings on login
- Only show "Delete playlist" for playlists the user owns
- "Add to playlist" submenu only lists owned playlists
- "Remove from this playlist" only appears when viewing an owned playlist

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/util/settings.hpp

@@ -21,6 +21,9 @@ public:
     QString displayName() const      { return m_settings.value("user/display_name").toString(); }
     void setDisplayName(const QString &n) { m_settings.setValue("user/display_name", n); }
 
+    qint64 userId() const            { return m_settings.value("user/id", 0LL).toLongLong(); }
+    void setUserId(qint64 id)        { m_settings.setValue("user/id", id); }
+
     // 5 = MP3, 6 = CD, 7 = HiRes96, 27 = HiRes192
     int preferredFormat() const      { return m_settings.value("playback/format", 6).toInt(); }
     void setPreferredFormat(int f)   { m_settings.setValue("playback/format", f); }